The earth's magnetic field provides a useful source of information which can, in principle, be used to help an animal to orient, migrate or to retun to its home territory. Behavioral evidence from bacteria, flatworms, molluscs, insects, fish, birds, and at least one mammal indicates that some organisms do indeed use this source of information. However, the fundamental questions about how this sensory information is obtained and processed in neural terms remain essentially unanswered. This proposal is motivated by our recent finding that the marine nudibranch mollusc Tritonia diomedea has a well-developed behavioral magnetic sense. We have found that this benthic opisthobranch tends to locomote on a predictable compass heading when deprived of other sensory cues. Tritonia is useful model system for studies of the neural basis of behavioral phenomena because: (i) its nervous system is composed of a relatively small number of large, pigmented and reidentifiable neurons some of which can be studied individually from animal to animal, (ii) it is feasible to study the neural mechanisms of relatively complex behavioral acts in the nearly intact animal using both intracellular and extracellular techniques and, (iii) previous work has established the functional roles of a number of neurons, and in particular, neural circuits associated with locomotion and sensory processing. In this proposal we describe experiments designed to (i) confirm further the nature of the behavioral magnetic sense, (ii) isolate and identify the primary sensory organs for geomagnetism and (iii) determine the mechanisms involved in the flow of sensory information about magnetic fields to the neuronal systems that control direction and rate of locomotion.